engleski

Long-term changes of the trophic levels of the Butoniga reservoir (Istria, Croatia)

In the paper the results of the longitudinal research of trophic features of the Butoniga reservoir, situated in the middle of the Istria peninsula, will be presented. The research started in 1987, i.e. when the reservoir was formed, and lasted till 200i. The reservoir served as a supplier of drink water and as a ban against floods. The reservoir has the following characteristics: a surtace is 2.3 km3, a volume is of 19*106 m3, a mean depth is 4.5 m, a maximum depth is 11 m and catchments area is 73 km2. It is supplied with water from 3 streams and by inflows what makes possible to change the whole amount of the water twice a year. It has to be pointed out that the water level oscillates more than 4 m. During the mentioned period of time physical-chemical characteristics (temperature, Secchi depth, dissolved oxygen, nutrient concentrations, chlorophyll a), the structure and abundances of zooplankton, phytoplankton, zoobenthos, macrophytes and fish populations were monitored. On the basis of the analyses it has been established that in the period 1987-1990 the reservoir was mild eutrophic, but after that the trophic level was kept on the mezotrophic level. It was mostly due to breakdown of organic materials after filling up of the reservoir. However, since 1998 monitored parameters has been showing the trend of increasing of the atrophic degree, as a consequence of the inflow of nutrients and sediment from a catchments area and uncontrolled introduction of fish of Cyprinidae family (a carp Cyprinus carpio and a prussian carp Carassius auratus gibelio). On the basis of the results of the researches the programme of measures to preserve and improve of water quality of the reservoir as a very important source of drink water has been proposed and accepted. Two types of measures were proposed: mechanical and biological. Among mechanical measures the following ones should be mentioned: resolving the inflow of nutrients and sediment from a catchments area by means of supplying streams, the outflow of lower levels of water at the end of summer and mechanical removal of submerge vegetation. Among biological measures the following was proposed: introducing of planktivorous fish, especially phytoplankton grazing fish (a silver carp Hipophthalmichthys molitrix and predatory fish species (a pikeperch Shiostedion lucioperca) and elimination previously mentioned undesirable fish species.

trophic features; water quality; Butoniga reservoir

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